1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for releasing a blank zone of a femto base station by a macro base station in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
An 802.16m system can use a technique for a femto base station (BS), and researches related to this technique are actively ongoing in recent years. The femto BS represents a very small-sized mobile communication BS used in an indoor environment such as homes, offices, etc. The femto BS has a similar concept with a pico-cell. Yet, the femto BS has a concept of having a more advanced function than the pico-cell. The femto BS generally has lower transmission power, and provides an access to either a subscriber or a subscriber group consisting of access providers. The femto BS is connected to an Internet protocol (IP) network deployed in homes or offices, and provides a mobile communication service by accessing to a core network of a mobile communication system. That is, the femto BS is connected to the core network of the mobile communication system through a broadband connection such as a digital subscriber line (DSL). Further, the femto BS can communicate with a macro BS overlaid with the femto BS by exchanging a control message through an air-interface. A user of the mobile communication system may receive a service via a conventional macro BS in an outdoor environment, and may receive a service via the femto BS in an indoor environment.
The femto BS improves indoor coverage of the mobile communication system by compensating for a problem in that the conventional macro BS provides a service with poor quality inside a building. Since the femto BS can provide a service only to a specific designated user, a voice service and a data service can be provided with high quality. Further, by reducing a cell size, a next generation cellular system using a high frequency band can have an increased efficiency, and since several small-sized cells are used, there is an advantage in that a frequency reuse number can be increased. Furthermore, the femto BS can provide a new service which is not provided by the macro BS. With the deployment of the femto BS, fixed-mobile convergence (FMC) has been sped up, and industry-based costs can be reduced.
The femto BS generally operates within an authorized spectrum. A frequency band used by the femto BS may be the same as or different from that used in the macro BS. The coverage of the femto BS may overlap with the coverage of the macro BS. The femto BS may provide a service to general users located inside a hot spot, or may provide a service to a closed subscriber group (CSG) which is a group of subscribers authorized by the femto BS or a service provider. When the femto BS provides a service to the CSG, a mobile station located close to the femto BS may be interfered by the femto BS in a downlink scenario, or may causes interference to the femto BS in an uplink scenario.
Accordingly, there is a need for a method capable of effectively decreasing interference between a macro BS and a femto BS.